OCULAE ILLUSIONS. 11 



and through which it transfers to that material 

 tablet its decisions and its creations. The eye is 

 consequently the principal seat of the super- 

 natural. When the indications of the marvellous 

 are addressed to us through the ear, the mind 

 may be startled without being deceived, and 

 reason may succeed in suggesting some probable 

 source of the illusion by which we have been 

 alarmed : but when the eye in solitude sees before 

 it the forms of life, fresh in their colours and 

 vivid in their outline ; when distant or departed 

 friends are suddenly presented to its view ; when 

 visible bodies disappear and reappear without any 

 intelligible cause ; and when it beholds objects, 

 whether real or imaginary, for whose presence no 

 cause can be assigned, the conviction of super- 

 natural agency becomes, under ordinary circum- 

 stances, unavoidable. 



Hence it is not only an amusing but a useful 

 occupation to acquire a knowledge of those causes 

 which are capable of producing so strange a 

 belief, whether it arises from the delusions which 

 the mind practises upon itself, or from the dex- 

 terity and science of others. I shall therefore 

 proceed to explain those illusions which have 

 their origin in the eye, whether they are general, 

 or only occasionally exhibited in particular per- 

 sons, and under particular circumstances. 



There are few persons aware that when they 

 look with one eye, there is some particular object 

 before them to which they are absolutely blind. 

 If we look with the right eye, this point is always 

 about 15 to the right of the object which we are 

 viewing, or to the right of the axis of the eye or 

 the point of most distinct vision. If we look 



